For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.

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For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.

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Just a thought; shouldn't the "size of print" above be replaced by "size of enlargement". If printing full frame they are of course the same, but not with a selective enlargement.

IMO the easiest method is to ignore image size and use enlarger lens to baseboard distance. This way selective enlargement or not, you are always measuring the same thing. You are never going to get caught out by not noticing that you have masked off the image etc.

Say you do a 10x8 ish print and the distance is from lens to baseboard is 50cm
Then you want to do a bigger print and having racked up the column and getting it how you want it the distance is now 70cm

70 squared (4900) divided by 50 squared (2500)= 1.96

If your exposure was 10 seconds for the first print it will be 19.6 for the second.

Simple - just keep a tape measure and calculator next to your enlarger. Takes a few seconds and never fails. Bear in mind that if exposures change a fair bit the relationship is not quite linear because lamps get hotter ie if the factor is 4 so goes from 5 seconds to 20, I find that with my enlarger the second time would be somewhat less than this, maybe 19 seconds. To fix this I tend to make small images at say f11 and the bigger ones at say f8 or f5.6. This way I can use aperture changes to help keep exposure time close. You can of course use a combination of both. Say the factor is 2.6 and you originally used 10 seconds at f11. This would be 26 seconds at f11......or you could use 16 seconds f8.

be sure to measure between the exact same two points on both the negative and the print. I will measure a particular distance on the negative before I place it in the carrier, then i can calculate the magnification easily when enlarging by measuring the same two points on the easel.

One more way to skin the cat - measure the size of the neg holder opening (always stays the same). When you get the magnification you want, measure the uncropped image at the easle plane. Should give accurate image enlargement ratio, I think(works for me). If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

........If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

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Yes, it’s the Inverse Square Law relating to light spread; hence the squaring referred to in Tom’s post above.

davetravis,
if you like to know the exact magnification ratio of an arbitariy negative clip to an arbitraty print size, simply replace the negative by a transparent ruler and measure the projected scale.

This is one thing I have never thought about. I just adjust height for the crop on the print size. Magnification factors have never been a concern to me. Than again I havent done anything under 4x5 for years.

vet173,
this could be of interest for several reasons, which do not necessarily have to do with enlarging itself. It could be necessary to know the scale of a reproduction or to produce an exact 1:1 reproduction. Of course, you'll have to know the exposure mag ratio in this case as well.

Thanks for the info. I use an ilford exposure meter for exposure determination. It's only an obsession with me not a job, so I don't do any product photography. I can see now where someone might need that kind of information.

I needed to know the mag factor so I could decide if I wanted to switch to an APO lens. My Rodagon 80mm is best around 6X. My 16x20's come to around that. My 20x24's are around 9X. The APO Rodagon is best around 10X.
Since the detail and contrast that I'm getting now printing on Ilfochrome is so fantastic, I have decided that the APO would be a waste of money for my current format. Besides, Ciba doesn't need any more contrast!
Thanks to all for responding.

Try the new app enLARGE available from the Apple App Store for iOS (iPhone, iPad). It lets you accurately determine the exposure time needed to make an enlargement of any size (eg a big enlargement) once you know the exposure time needed to expose an enlargement of any one size (eg a tiny work or test print)

Okay, two questions: a) How do you measure the difference in size between them (and how accurate are your measurements?) and b) Having done so, what do you do with the two measurements? How do they help you to determine the exposure time? Thanks

One more way to skin the cat - measure the size of the neg holder opening (always stays the same). When you get the magnification you want, measure the uncropped image at the easle plane. Should give accurate image enlargement ratio, I think(works for me). If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

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Measuring or determining the magnification change will not, in itself, allow you to accurately recalculate an exposure difference. There are other considerations which introduce significant exposure error, the main one being that the internal components of the lamp house (most importantly the distance between the lamp and the negative) wrongly remains fixed in position when they (or it) should be changing (lengthening or shortening) with a change in magnification. That’s why area and magnification calculations don’t work accurately enough to make them worthwhile. The enLARGE app (for iPhone) gets around this problem by measuring the relative light output of the enlarger at different print magnifications at the paper plane itself using the photo paper as the measuring and calibration tool; effectively, magnification becomes irrelevant by this method.

Magnification adjustment factors are only of interest if you wish to change your enlargement size without producing a second test print(s). Negative size is irrelevent.

Click to expand...

Measuring or determining the magnification change will not, in itself, allow you to accurately recalculate an exposure difference. There are other considerations which introduce significant exposure error, the main one being that the internal components of the lamp house (most importantly the distance between the lamp and the negative) wrongly remains fixed in position when they (or it) should be changing (lengthening or shortening) with a change in magnification. That’s why area and magnification calculations don’t work accurately enough to make them worthwhile. The enLARGE app (for iPhone) gets around this problem by measuring the relative light output of the enlarger at different print magnifications at the paper plane itself using the photo paper as the measuring and calibration tool; effectively, magnification becomes irrelevant by this method.

I have a Beseler 23C XL II, printing 6x7 cm with an 80mm Rodagon.
Does anyone know what the "factors" are for 11x14, 16x20 & 20x24?
Thanks.

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A new enlarging app called enLARGE (available on Apple AppStore for iPhone) lets you calculate exact equivalent exposure times for any and all enlargement sizes, not just preset or standard sizes, and it lets you do it simply by measuring the distance between the print and negative planes using a tape measure - no need to measure the size of the projected image at the print plane.

the internal components of the lamp house (most importantly the distance between the lamp and the negative) wrongly remains fixed in position when they (or it) should be changing (lengthening or shortening) with a change in magnification.

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The OP is making color prints. Probably he is using a diffusion enlarger.

A fascinating array of solutions.
I find something that is one inch on the first print and then measure it on the second print, square the number and that is my factor. But it is usually approximate particularly as it is very likely I will change contrast as well.

"There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

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